Electronic tube



P. K. WEIMER ELECTRONIC TUBE Jan. 9, 1951 3 Sheets-Sheet 1 Filed Aug. 5,1945 I ENTOR.

BY w/m azw/ Jan. 9, 1951 P. K. WEIMER ELECTRONIC TUBE Filed Au 5, 1945 3Sheets-Sheet 2 I'll INVENTOR. ME

Wail/la firm/Mi) landing on the target remains the .same.

Patented Jan. 9, 1951 ELECTRONIC TUBE Paul K. Weimer, Princeton, N. .L,assignor to Radio Corporation of America, a corporation of DelawareApplication August 3, 1945, Serial No. 608,663

1 12 Claims.

This invention relates to television pick-up tubes, particularly thosehaving two-sided targets. A suitable type of two-sided target haspreviously been developed which consists of a thin sheet ofsemi-conducting glass having such thinness and conductivity that acharge image formed on one side will be discharged'within a frame timeby electrons landing from a beam scanning the other side. The glass, onthe other hand, is sufiicienty non-conducting to prevent the spreadng ofthe charge image to a material extent within a frame time. A televisionpick-up tube having such a target is disclosed in the application ofAlbert Rose, filed November 28, 1945. Serial No. 631,441, which is nowU. S. Patent 2,506,741, granted May 9, 1950 and an improved type isdisclosed in' the applicat on of Harold B. Law, filed April 19, 1945,Serial No. 589,241, new U. S. Patent 2,460,093 issued January 25. 1949.This type of tube is generally known as the image orthicon.

The image orthicon is particularly suited for electron multiplication ofthe modu ated cathode beam returning from the region of the target and Ihave developed an efficient electron multiplier for use in this type oftube, .on which application was filed September 16, 1944, Serial No.

554,494,.now U. S. Patent 2,433 ,941, issued January 6,1948.

In the image orthicon, as disclosed in the foregoing applications, acharge pattern is formed on the unscanned side of a thin glass target,which sets up a potential pattern on the scanned side of the target. Asthe beam is scanned at substantially zero velocity over the target in anelectro-magnetic focusing field, sufficient electrons land to bring theelemental areas down to cathode potential as a potential datum and theremainder of the beam returns to the multipliers.

This manner of discharging the target image has certain disadvantages,an important one .being the limitation of signal-to-noise ratio. Thesignal resulting from this type of discharge is proportional to theelectrons landing on the tar-'- get from the beam for discharge oftheimage; The beam must have sufiicient intensity to furnish suflicientelectrons vior this purpose, but increas ing the intensity above thisvalue does notin crease the signal, as the number of electrons On theother hand, noise in apick-up tube is proportional to the square root ofthe beam current; therefore, low signalto-noise ratio in the prior typesof image orthicon is a necessary .consequence of small beam current.

Another object is to provide a pick-up tube in which more charge can helanded from the beam than is stored on the target.

Another object of the invention is to provide an image orthicon in whichthecharge image on the target is discharged by means other than thescanning beam.

Another object of the invention is to provide an image orthicon in whichthe potential controllng the landing of the electrons from the cathodebeam adjacent the charge image is constant while an elemental area isbeing scanned.

Another object .of the invention is to provide a tube with a two-sidedtarget that is discharged independently of the cathode beam.

Another object of the invention is to provide a target and associatedelements so that signals proport onal to the charge image may begenerated without removing the charge image there-'- by Further objectsof the invention will appear in the following description, referencebeing had to the drawings, in which:

Fig. 1 is a .diagrammatical illustration of an image orthicon containingmy improved pick-up tube with its operating'circuit.

Fig. 2 is an enlarged section through a very small area of the controlscreen indicating the equipotential lines of the electrostatic field inthe vicinity of the scanned side of the target screen;

Fig. 3 is graph of current produced by the electrons landing on thedischarge screen vs. the

potentials of the target.

Fig. 4 illustrates how my invention may be used to produce acharge-image amplifier.

Fig. 5 is a modification of the target of Figure 4.

Referring to Fig. 1, the improved image orthicon consists of anevacuated envelope l containing a gun 2 at one end thereof with a glassanode are partially sectioned to illustrate their tubular construction.The grid and first anode have minute orifices, as indicated, throughwhich the beam 9 is projected in the usual Way to reach the target withapproximately zero velocity. Tubular electrode I is axially placedaround the front part of the gun adjacent thereto. This is oftenreferred to as a persuader, because its function is to direct thesecondary electrons I emitted by the dynode 8 into the succeedingmultiplier dynode, as will be hereinafter referred to. Wall coating IImay be applied to the inner Wall of envelope I in known manner. 7

Outside the envelope I is placed the deflecting unit I2, consisting of aframe having two electromagnetic coils with their field axesperpendicular to each other and to the longitudinal axis of the tube.One of these coils deflects the beam in a vertical direction in Fig. land the other deflects it at right angles to the plane of the drawing.These coils are of well-known construction and hence have not beenindividually shown. It will be understood that the deflection coils willhave varying voltages applied thereto, say by saw-tooth generators ofsuitable frequencies, to produce line and field scansion. The deflectingunit I2 is made manually adjustable by suitable means, such as a smallrod I3, for balancing the electrostatic and magnetic forces for uniformlanding on the target, as disclosed in my copending application, filedNovember 28, 1945, Serial No. 631,440, which is now U. S. Patent2,533,073, granted December 5, 1950.

Outside of the envelope I is compensating coil I4 having a'field'perpendicular to the axis of the tube. By adjusting this coilaround the tube axis, any slight miscenteringof the beam due tomechanical imperfections can be eliminated. Also, outside the coils I2and I4 is placed coil l5, which produces a strong magnetic focusingfield parallel to the axis of the tube on both sides of the target.

Around the gun is placed a plurality of ad ditional multiplier dynodesI6, I1, I8 and I9 and a coll cting electrode 20, which is a screen. Thisis connected to the desired utilization device indicated as an amplifiertube 20. The multiplier dynodes and the other parts of Fig. 1 thus farreferred to are particularly described in my said application and itwill be sufiicient' to say respecting the dynodes I6, I! and I8that'each consists of angular radial blades 2!, somewhat like anelectrical fan, held in a suitable annular frame, having a suificientaxial opening to pass in non-conducting relation over the first anode l.Multipliers of the type disclosed in my copending application, filedAugust 21, 1945, Serial No. 611,720, now U. S. Patent 2,443,547 issuedJune 15, 1948 or any other type, may also be used, as my invention isnot limited 'to any par ticular type of multiplier.

In front of the blades of the dynodes shown are screens 22 secured tothe annular frames so as to be in electrical contact with the associatedmultiplier blades 2|, from which they are suitably spaced. Themultiplier dynodes I6, I? and I8 are partially broken away toshow theblades 2I and screens 22. The multiplier dynode I9 is the finalmultiplier stage and consists of a flat annulus spaced from andsurrounding the first anode, similar to the other dynodes. Themultiplier dynodes 8, I6, I'I, I8 and I9 may have any desired coating ofactive material to produce suitable emission of secondary electrons upon4 bombardment by primary electrons. Various voltages may be applied tothe dynodes and other electrodes of the tube. By way of example I haveindicated in the drawing suitable voltages.

At the opposite end of the tube, the photocathode 4 may be asemi-transparent coating of light-sensitive emissive material on theinside of the end of the envelope I. A frame 23 of metal supports thethin glass target 3, such as disclosed in the said Rose application, andalso supports the mesh collecting screen 24 spaced from the'tanget,preferably as disclosed in the said Law application, but it may bespaced and supported in any other desired way. The various electrodeswould be supported in well-known ways, not illustrated.

Adjacent the glass target sheet 3 is arranged the decelerating ring 25,which is grounded to gun cathode potential or thereabouts. Between theframe 23 and the photocathode 4 is electrode '26, which is connected toa potential more negative than ground, but having sufiicient positivepotential relative to that of the photocathode I to accelerate at highelectron velocity the photo-electrons onto the glass target sheet 3along the magnetic lines of the focusing coil I5. Suitable voltages forelectrode 26 and photocathode 4 are indicated in the drawing.

The parts thus far described in Fig. 1 are disclosed in my saidapplication 554,494 now Patent 2,433,941 and are not, per se, claimedherein.

In the orthicon of my said application, the charge pattern formed on theunscanned side of target 3 is discharged by the electrons landed frombeam 9 on the scanned side of the target, which brings it to thepotential of the cathode as a datum potential. In the invention of thisapplication, the electrons from the beam do not land in substantialquantity on the target 3. The potential pattern thereon does, however,control the landing of beam electrons on adjacent metal mesh screen 21and hence modulates it in'accordance with the potential pattern. This isaccomplished without discharge of the chargeimage on the unscannedsurface of target 3. The discharge screen 21 is substantially on thesurface of glass target 3 but spaced therefrom on the gun side'sufiiciently to prevent conductive contact. This mesh screen 21 has thepotential of the cathode of the gun or slightly positive thereto. Thecollecting screen 24 controls the average potential of the glass target3 and may be biased from 2 to 6 volts or more negative to the dischargescreen 21.

Fig. 3 shows the average potential of the collecting electrode 24relative to the current produced by the electrons landing on the meshdischarge screen 21. For one method of operation, the negative bias ofmesh screen 24 is adjusted so that the variation in potential due to thecharge image may fall on the part A--B of the curve of this figure andfor another method, operation may be had on the portion of the curve CD.

The theory of operation will first be explained when the bias voltage ofmesh screen 24 is such that operation is had on the part A-B of thecurve of Fig. 3.

' Suppose a moving scene is focused on the photocathode I (Fig. 1).Photo-electrons are emitted and are projected at high velocity along themagnetic focusing lines of coil I5 through mesh screen 24 onto theunscanned side of the glass target 3. A charge pattern is formed on thissurface of the glass 3, due to the emission of secondary electrons,whichare collected by mesh screen 24, leaving the target surfacevaryingly positive in proportion to the light density of the pictureprojected onto photocathode 4. The target has capacitative relation withdischarge screen 21 and the charge pattern sets up a potential near andaround the discharge screen. The bias voltage field applied to thecollectin screen 24 sets up a voltage around the screen 21, theequipotential lines of which are indicated :in Fig. 2. The charge-imageon the photocathode side of the glass target 3 varies these potentialsin proportion to the charge-image.

It will be seen from Fig. 2 that the negative bias voltage applied tothe collecting mesh screen 24, makes the path in front (gunside) of theopen meshs 28 of discharge screen 21 more negative than the pathlnfront-of the wires 29 of screen 21, which have zero potential difierencein respect to the gun cathode. The electrons from the beam, as theyapproach the glass target '3, find in their path the +0.4V potentialline, the zero potential line, the -0.4V, -;6V and -1.0V negativepotential lines. The electrons will be deflected to less negativepotentials and 29) that has a definite. potential/though they I reachthat electrode by passing through a potential field controlled by thecharge pattern. In my said prior application and in other prior artstructures, as far as I am aware, the electrode on which the beamelectrons land (glass target-3 or equivalent) changes its potential asthe electrons land.

It will be appreciated that the electrons landing on screen wires 29 donot discharge the charge pattern on the photocathode side of the target3. However, with a moving scene in the next one or more frames, as thelight changes, the secondary electrons emitted by photo-electronbombardment of the glass target will not all go to the collecting screen24, but enough will rain down on the elemental areas of the previouschargepattern to discharge the scanned elemental areas. Thus, in myimproved pick-up tube, the landing of photo-electrons produces thedischarge of the charge pattern by bombarding secondary electrons fromthe glass target that land on adjacent scanned areas in a frame. Thedischarge of the image by means other than the landing of beam electronson the target areashas the important advantage that urface conditions onthe screen electrode 21 may bevaried to control the effects of thelanding thereon without varying the effect of the charge-image on thebeam.

With stationary scenes the glass target will, of course, notbedischarged at all, as the photoelectrons bring the potential up on theelemental areas and counteract the raining down of Sec ondaries fromadjacentjareas, With stationary pictures the illumination thereforebuilds up to saturation, the charges in successive frames being added tothose of previous frames. Thus, in stationary pictures my invention isan improvement over the usual tube, in which the beam discharges thescannedv areas, in that greater charge-image is obtained'through'build-up. An-

important advantage of 'this' 'action is that, in taking stationarytelevision pictures with very low lighting, the full storage time of thetarget may be many times greater than a frame time. This permits one toconstruct a television pickup tube of extremely high sensitivity andtime e posure may be taken, as in photography, while continuouslytransmitting the picture.

Incase moving pictures are taken where the average light intensitysuddenly decreases, the secondaries produced by the photo-electrons maynot rain down in the surface of target 3 sufiiciently to discharge thecharge-image in the desired time. In that case, the negative potentialbias of screen 24 can be reduced so that some of the electrons of thebeam will land and help discharge the image.

The beam modulated by the potential-pattern of the charge-image returnstoward the gun to the first dynode 8, where the secondaries bombardedtherefrom are directed through screen 22 into the second dynode l6 underthe influence of the potentials in dynode l6 and persua'der it. Thesecondarie emitted by the vanes 2| of dynode l6 pass through screen 22to the plates of the third dynode I1 and a similar operation is carriedout in dynodes I 8 and i9. The greatly multiplied secondaries from thelast dynode i9 are collected by screen electrode 20 and utilized in thecircuit typified by amplifier tube 29.

If desired, the signal may be obtained from the discharge screen 21without multiplication and the multiplier structure omitted. This willbe particularly feasible in my improved tube, as the intensity of thebeam current can be increased 'tobring up the signal value, as thesignal-to-nois ratio is expressed in the ratio a L k f as alreadyexplained.

By decreasing the negative bias of screen 24 relative to screen .21,operation may be had on portion CD of the curve of Fig. 3. In thioperation, the charge-image is less negative to the screen 21 and thelight regions of the picture tend to attract beam electrons moredirectly toward the openings 28 ofthe screen. These, however, do notland but return to the multiplied dynodes. In the dark areas of thepicture, the electrons tend not to enter the openings 28, due to the:lower potential established by the charge-image and have a greatertendency to be defiected to the wires 29. Less beam current wouldtherefore return to the multiplier for the dark regions. With thismethod of operation, maximum beam current would reach the multiplier forwhite regions and minimum current for dark regions. This reversal ofpolarity produces an important result. It is well known that, in theusual image orthicon, noise is most noticeable and objectionable in thedark regions of a picture and myimprovement reduces the signal-tonoiseratio in the dark regions by reducing the beam current going to themultipliers. Another important result in this method of operation isthat the reduced beam current reaching the first dynode of themultiplier will reduce the spurious signal produced by thenon-uniformity of the 7 surface or other characteristics of the dynode,

7 on the glass target 3 which is a target for the photo-electrons.

A modified method of operation may also be had on the portion CD of thecurve by biasing the discharge screen 21 2 to volts positive relative tothe gun cathode (ground), so that secondary electrons are emittedtherefrom under the impact of the beam electrons. These secondaryelectrons will escape and accelerate to the multipliers in greaternumber when the beam is over the bright areas because the glass swingsless negative, that is, in the direction D, and less secondary electronswill escape from the screen indark areas when the potential of the glasstarget swings more negative, that is, toward C. In this modifiedoperation, substantially no beam electrons land on the relativelynegative glass target.

In Fig. 4 I have indicated how image amplification may be obtained witha modification of my improved target. The image produced on glass target3 would be produced by collecting on a collector screen 24" thesecondary electrons bombarded from the surface of glass target 3 by thephotoelectrons from photo-cathode 4. Discharge screen 21" would besensitized and light rays 30 would produce photo-electrons therefrom.Focusing coil l5 may be arranged in sections to permit this entrance oflight. The photo-electrons leaving the sensitized screen 2'! will begreater in number than the photo-electrons landing on the other oppositesurface of the glass target 3 and will be proportional to the potentialpattern of the charge-image produced by the lastmentionedphoto-electrons on target 3. Thus, the screen-excited arrangementconstitutes an amplifier stage. The amplified photo-electrons emitted byscreen target 21 will be accelerated and focused onto another glasstarget 3' through collecting screen 24' to. produce an amplified chargepattern thereon by emission of secondary electrons collected by screen24'. This will control' the photo-electrons emitted by sensitized screen21' and the photo-electrons from this second stage will be acceleratedand focused on a third glass target 3" through collecting screen 24 andproduce a further amplified charge-image. The operation from this pointon will be the same as already described. Thus, an amplified image canbe produced before scanning the beam 9 over the target.

The discharge screens 21 and 21" may be caused to emit electrons bythermal treatment under control of the charge-image and the action wouldbe the same as just described. For example, the screens 2'! and 21" mayconsist of strands 3| arranged so that a heating current from a source32 may be passed through them in parallel, as indicated in Fig. 5, andcause them to emit electrons under control of the potential pattern ofthe adjacent glass target. In this case, the focusing coil may be ofstandard construction instead of the form shown in'Fig. 4. Obviously,the strands of the screen may be arranged in series instead of inparallel. In the image amplifier of Fig. 4, it would be preferable tomake the glass target opaque to prevent scattering of light andreduction of contrast.

Various modifications may be made without departing from the scope ofthe invention.

Having described my invention, what I claim is:

1. An electron tube system comprising a tube having a target, a screenelectrode closely adjacent said target, a multiplier dynode relativelyremote from said screen electrode, means including'a photo-cathode forproducing achargeimage and a potential pattern on said target, apotential source biasing said target negative to said screen electrode,means for supplying electrons to a region closely adjacent elementalareas of said screen electrode, the potential of said pattern oppositeeach of said elemental areas causing one current of electrons to movefrom said region toward said screen electrode and another currentthereof to move toward said dynode, one of said currents being directlyproportional to said potential and the other being inverselyproportional thereto and means for producing a focusing field betweenthe screen electrode and the multiplier dynode.

2. An electron tube system comprising a tube having a target sheet, ascreen electrode closely adjacent one surface of said image screen, amultiplier dynode relatively remote from said surface, means including aphoto-cathode for producing a charge-image on the other surface of saidtarget sheet and producing potential pattern on the first-mentionedsurface, a potential source biasing said target sheet negative to saidscreen electrode, means for supplying electrons to a region closelyadjacent elemental areas of said screen electrode, the potential of saidpattern opposite each of said elemental areas causing one current ofelectrons to move toward said screen electrode and another current tomove toward said dynode, one of said currents varying directly with saidpotential and the other one varying inversely therewith, and means forproducing a focusing field between the screen electrode and the dynode.

3. A pick-up tube system comprising a tube having a cathode ray beamgun, a target sheet, an electrode adjacent one side of said targetsheet, and means for forming a charge-image on the other side of saidtarget sheet, a potential source applying substantially thesame'potential to said electrode and the cathode of said gun, means forscanning the beam of said gun over said electrode, and a. potentialsource biasing said target sheet sufiiciently negative to said electrodeto prevent substantial landing of beam electrons thereon.

4. A pick-up tube system, comprising a tube having a cathode ray beamgun, a target sheet, a mesh screen adjacent one side of saidtargetsheet, a photo-cathode spaced from the other side of said targetsheet, a potential source applying substantially the same potential tosaid screen and the cathode of said gun, electric field producing meansfor projecting photo-electrons from said photo-cathode onto said otherside of said target sheet to bombard secondary electrons therefrom, anelectrode between the photo-cathode and the target for collecting saidsecondary electrons, a potential. source connected to said electrodehaving a potential adapted to produce a bias on the targetsheetsufiiciently negative to the screen to prevent beam electrons landing onsaid target sheet and electric field producing means for scanning saidbeam over the screen for landing electrons thereon under control of thepotential set up by the emission of said secondary electrons.

5. A pick-up tube system comprising a.tube having a cathode ray beamgun, a dielectric target, a first mesh screen adjacent one side of saidtarget connected to the cathode of said gun to have substantially itspotential, a photo-cathode on the other side of said dielectric target,field producing means for projecting photo-electrons to said firstscreen to prevent substantial landing of beam electrons on said targetthrough the meshes of said first screen without blocking their landingon the strands thereof under control of said potential pattern, andfield producing means for scanning said beam over said first screen.

6. A pick-up tube system comprising a tube having a cathode ray beamgun, a multiplier dynode positioned around the axis of said gun andadjacent the front end thereof, a dielectric target, a mesh screen atone of the sides of said dielectric target connected to the cathode ofsaid gun to have substantially its potential, a photocathode and fieldproducing means on the other side of said target for projectingphoto-electrons thereonto at sufficient velocity to bombard secondaryelectrons therefrom and produce a potential pattern on thefirst-mentioned side, an electrode between the photo-cathode and saiddielectric target for collecting said secondary electrons, a potentialsource connected to said electrode adapted to set up a sufl'icientlynegative potential in the meshes of the screen to prevent substantiallanding of beam electrons on said target and permit their landing onsaid screen under control of said charge-image, field producing meansfor scanning said beam at substantially zero electron velocity over saidmesh screen, field producing means forming a uniform electromagneticfocusing field axially of said gun, said gun accelerating the electronsof the'beam not landing on said screen along lines of said focusingfield to land on said dynode with suflicient electron velocity tobombard'secondary electrons therefrom.

7. A pick-up tubesystem comprising a tube having a cathode ray beam gun,dielectric target, a mesh screen at one of the sides of said dielectrictarget connected to the cathode of said gun to have approximately itspotential, a photocathode and field producing means for forming acharge-image on the other one of said sides of said dielectric target,said charge-image setting up a potential pattern on the first-mentionedside, field producing means for scanning said beam in front of theelemental areas of said target, an electrode, a potential sourcenegatively biasing said target relatively to said mesh screen forlanding beam electrons on the mesh screen at a rate varying inverselywith the decrease of said negative bias by said potential pattern.

8. A pick-up tube system comprising a tube having a cathode ray beamgun, a target structure including a glass sheet and a mesh screen at oneof the sides of said glass sheet, means connecting said screen to thecathode of said gun to have approximately its potential, a photo cathodeand field producing means for forming a charge-image on the other one ofsaid sides of said glass sheet setting up a potential pattern on thefirst-mentioned side, field producing means for scanning said beam infront of the elemental areas of said screen, an electrode, a potentialsource negatively biasing said glass sheet relatively to said meshscreen for landing beam electrons on the mesh screen at a rate varyingdirectlywith the decrease of said negative bias by said potentialpattern.

9. The method of operating a cathode ray beam pick-up tube having a gun,a dielectric target, a mesh screen adjacent the gun side of saiddielectric target, said method comprising the steps of applying apotential to the mesh screen substantially equal to the potential of thecathode of the gun, biasing the dielectric target sufficiently negativeto the mesh screen to prevent the electrons of the beam from landing onthe dielectric target and producing a charge-image on the dielectrictarget to varyingly decrease said negative bias to land electrons of thebeam on the mesh screen under'control of the potentials set up by saidcharge-image.

10. The method of operating a cathode ray beam pick-up tube having agun, a dielectric target sheet and a mesh screen closely adjacent oneside of the dielectric target sheet, said method comprising the steps ofapplying a potential to the mesh screen substantially equal to thepotential of the cathode of the gun, producing a charge-image on thedielectric sheet, scanning the beam over the mesh screen, biasing thedielectric sheet sufficiently negative to prevent electrons landingthereon and to cause them to land on the mesh screen in proportion tothe charge-image.

11. The method of operating a discharge tube having a dielectric targetsheet and a mesh screen closely spaced from and overlying one surface ofsaid dielectric sheet, said method comprising the steps of establishinga charge pattern on said dielectric sheet, establishing a potential onthe mesh screen, providing a flow of electrons away from the meshscreen, and biasing the dielectric sheet sufficiently negative relativeto the potential of the mesh screen to control the electron flow awayfrom areas of the mesh screen in proportion to the charge on theunderlying portion of the dielectric sheet.

12. The method of operating a discharge tube having a dielectric targetsheet and a mesh screen closely spaced from and overlying one surface ofsaid dielectric sheet, said method comprising the steps of, establishinga charge pattern on said REFERENCES CITED The following references areof record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,176,190 Schroter Oct. 17, 19392,254,617 McGee Sept 2, 1941 2,407,906 Rose Sept. 17, 1946 2,433,941

Weimer Jan. 6, 1948

